This invention pertains generally to microwave oscillators and more particularly to a low-noise microwave oscillator.
It is well known that many advanced electronic systems for communication and navigation require circuits, including oscillator circuits, having low-noise for low noise applications. Such low noise applications including space communications, electronic warfare countermeasures and modern radar techniques. In a microwave oscillator, frequency modulation (FM) noise is a problem due to the noise characteristics of the components of the oscillator. In many applications, it is advantageous to reduce the FM noise characteristics of the microwave oscillator. For example, lower noise is a key to extending the maximum detection range of a radar missile. Various techniques for lowering the FM noise of a microwave oscillator have been described. For example, a technique for degenerating low frequency FM noise arising within a microwave oscillator is described in U.S. Pat. No. 5,032,800, issued Jul. 16, 1991, entitled "Tunable Oscillator with Noise Degeneration" (which patent is assigned to the same assignee as this application). In said Patent, it is described that a microwave oscillator includes an amplifier and a feedback circuit disposed about the amplifier. The feedback circuit includes a resonator having a first port and a second port and a voltage-controlled phase shifter having a input port, an output port and a control port, the input port of the voltage-controlled phase shifter connected to the output port of the amplifier and the output port of the voltage-controlled phase shifter coupled to a port of the resonator. The oscillator further includes a circuit responsive to signals from the output of the voltage-controlled phase shifter and the first port of the resonator to provide a control signal to the control port of the voltage-controlled phase shifter for degenerating low frequency FM noise arising with in the amplifier.
Although such a technique is useful in many applications, the noise performance of a microwave oscillator is a very important consideration and lower noise is a key to extending the maximum detection range of a radar. For example, in a doppler radar, noise generated at base band frequencies is upconverted and occurs in the region from the carrier at the order of expected doppler frequency shifts and will reduce the subclutter visibility of the radar. Therefore, it is desirable to provide a very precise, yet easily changeable microwave frequency signal source with a highly pure stable frequency output signal.